1. Field of the Invention
The present invention is directed to a method of forming a complex profile defined by a number of even depressions in the surface of a workpiece, and more particularly to such a method of using an energy beam for providing a microstructured profile in the surface of an ablatable workpiece.
2. Description of the Prior Art
The use of an energy beam has been proposed to producing a desired microstructure profile in a workpiece by ablation, as disclosed in U.S. Pat. Nos. 4,128,752 and 4,842,782. In order to give a complex profile of uneven depressions distributed over a wide range of the surface, the energy beam should be precisely controlled in combination with a mask of complicate configuration. However, the mask of complicate configuration is difficult to fabricate and is only possible at a considerably high manufacturing cost. Hence, there is a demand to realize the complex profile in the surface of the workpiece expeditiously in a cost effective manner.
The present invention has been achieved to satisfy the above demand and provides a method of forming a complex profile of uneven depressions in the surface of the ablatable workpiece with the use of a simple optics or a mask of a simple configuration. The method comprises the steps of determining the complex profile in accordance with a particular feature to be given to the surface of the workpiece, then dividing the complex profile into more than one simple and regular waveform patterns of different characteristics, and irradiating an energy beam to the surface of the work piece to form the individual regular waveform patterns successively in an superimposed fashion by ablation in the surface of the workpiece. Accordingly, the desired complex profile can be easily obtained at an improved efficiency and at a moderate cost.
Preferably, the complex profile is divided into the regular patterns of sinusoidal waveforms by approximation using an orthogonal transformation, such as by the Fourier analysis.
A mask is utilized which includes a plurality of concentric opaque annuluses so as to image a one-cycle fraction of the regular waveform pattern on the workpiece by diffraction of the energy beam passing through the mask and an optics. The workpiece is shifted relative to the mask and optics to make the one-cycle fractions continuous to realize the regular waveform pattern. Another regular waveform pattern may be imaged successively on the workpiece by varying a magnification of the optics with the use of the same mask or by the use of a separate mask of different characteristics. Further, it is preferred to use a mask including an array of masking units each comprising the concentric opaque annuluses.
Further, it is preferred to adjust beam intensity to give a greater beam intensity directed towards sidewalls of the depression being made than towards a tip and a valley of the depression in order to precisely form the sinusoidal wave pattern.
Further, a deposited mask on the surface of the workpiece may be utilized for imaging one of the regular waveform patterns on the workpiece. The deposited mask includes an array of masking units each comprising a plurality of adjoining masking annuluses of different degrees of transparency to the energy beam arranged concentrically around a center opening. The energy beam incident to the deposited mask is controlled to image another regular waveform pattern in the absence of the deposited mask is irradiated through the deposited mask to form the complex profile. Thus, the complex profile can be readily realized by a single operation of scanning the energy beam onto the surface of the workpiece.
Instead of using the mask, it is possible to use the optics includes a special lens which distributes the energy beam into a plurality of beam spots on the surface of said workpiece each having a regularly varying beam intensity. The beam spots are arranged uniformly in an array so as to define any one of the regular waveform patterns by the array of the beam spots. The optics is adjusted to vary its magnification to image one of the regular waveform pattern firstly at a first magnification on the surface of the workpiece and then image another waveform pattern on the same surface at the second magnification. Thus, the desired complex profile can be realized with two or more steps of imaging the waveform patterns at varying magnification on the surface of the workpiece.
The energy beam is preferably a laser beam of a suitable pulsation frequency. The surface of the workpiece may be scanned by moving the laser beam in synchronism with the pulsation frequency to provide the complex profile over a wide range. Alternatively, the workpiece may be moved with the optics being fixed to provide the complex profile over a wide range. The laser beam utilized in the method of the present invention is preferred to have a short wavelength and/or short pulse width in order to enable precise laser ablation.
These and still other objects and advantageous features of the present invention will become apparent from the following description of the preferred embodiments when taken in conjunction with the attached drawings.